Circulating lymphocytes traffic among the blood vasculature, the lymphatic system, and sites of chronic inflammation facilitating interactions among lymphocytes, antigens, and accessory cells that ultimately lead to the generation and dissemination of an immune response. Entry of blood-borne lymphocytes into the lymphoid organs entails adhesion to the postcapillary endothelia followed by extravasation. For reviews, see: Jalkanen, S. T., et al., Immunol. Rev. 91:39-60, 1986a; Woodruff, J. J., and L. M. Clarke, Ann. Rev. Immunol. 5:201-222, 1987; Gallatin, M., et al., Cell 44:673-680, 1986. In peripheral lymph nodes, mucosal lymphold organs (Peyer's patches and appendix), and inflamed synovia, this adhesion-mediated entry occurs primarily at specialized high-walled endothelial cells lining the postcapillary venules (HEY). Adhesion and possibly transmigration across the venule is postulated to be mediated by a specific lymphocyte surface receptor or receptors interacting with complementary HEV molecules. These lymphocyte adhesion receptors (also referred to as homing receptors) have been implicated in the interaction of other nonlymphold hematopoietic cells with vascular endothelia (Lewinsohn, D. M., et al., J. Immunol. 138:4313-4321, 1987), and are postulated to play a role in the metastasis of lymphold tumors (Jalkanen, S. T., et al., 1986a, supra). There is mounting evidence, discussed below, that these adhesion receptors may function widely in other tissue systems as well.
Studies conducted both in vivo and in vitro revealed that subsets of lymphocytes preferentially migrate to or adhere to the HEV of different lymphold organs, suggesting the involvement of multiple adhesion receptors with different specificities. Griscelli, C., et al., J. Exp. Med. 130:1427-1451, 1969; Guy-Grand, D., et al., Eur. J. Immunol. 4:435-443, 1974; Scollay, R., et al., Nature 260:528-529, 1976; Smith, M. E., et al., Monogr. Allergy 16:203-232, 1980; Butcher, E. C., et al., Eur. J. Immunol. 10:556-561, 1980; Stevens, S. K., et al., J. Immunol. 128:844-851, 1982. Some lymphoid tumors were found to express a unispecific preference for the HEV of either peripheral lymph node or for gut-associated lymphoid tissue (Butcher, E. C., et al., 1980, supra). These data suggest the participation of at least two distinct adhesion receptors that confer lymphold organ specificity, contributing to the migratory patterns of lymphocytes.
Direct evidence for the presence of multiple adhesion receptors emerged in rodents, humans, and nonhuman primates as immunological reagents recognizing these molecules became available. MEL-14 is a monoclonal antibody raised against a 90 kD cell-surface protein present on a peripheral node HEV-binding mouse lymphoma, 38-C13 (Gallatin, W. M., et al., Nature 304:30-34, 1983). The mouse protein defined by MEL-14 is glycosylated and ubiquitinated, and contains internal disulfide bonds (Siegelman, M., et al., Science 231:823-829, 1986; St. John, T., et al., Science 231:845-850, 1986). MEL-14 reactivity correlates with peripheral node HEV-binding specificity of B and T cell tumors; and when either normal lymphocytes or unispecific tumor cells are pretreated with the MEL-14 antibody, in vitro adhesion to peripheral node but not Peyer's patch HEY is blocked, and migration in vivo to peripheral nodes is selectively diminished (Gallatin, W. M., et al., 1983, supra).
Monoclonal antibodies (mabs) that identify glycoproteins with similar functions in primates have also been characterized: Hermes-1, which recognizes migratory competent and HEV-adherent human lymphocytes (Jalkanen, S. T., et al., Eur. J. Immunol. 16:1195-1202, 1986b); Hermes-3, which specifically blocks lymphocyte binding to human appendix and Peyer's patch HEV (Jalkanen, S. T., et al., J. Cell Biol. 105:983-990, 1987); and Hutch-1, which defines related molecules in macaques (W. M. Gallatin, unpublished data). All of the epitopes recognized by these mabs reside on the same molecule. Although the MEL-14 and Hermes-1 receptors are apparently immunologically related (Jalkanen, S. T., et al., 1987, supra; Jalkanen, S., et al., J. Immunol. 141:1615-1623, 1988), no direct evidence indicates that they are the products of homologous genes.
Biochemical similarities between the Hermes/Hutch class of adhesion receptors and the class III extracellular matrix receptor (ECMRIII), a molecule postulated to function as a transmembrane link between the extracellular matrix and the cytoskeleton (Carter, W. G., and E. A. Wayner, J. Biol. Chem. 263:4193-4201, 1988), led to a detailed comparison of the two receptors (unpublished data; T. P. St. John, W. M. Gallatin, et al.). These studies identified extensive structural homology between these molecules. The tissue distribution of ECMRIII is quite broad, including granulocytes, monocytes, fibroblasts, several epithelial carcinomas, as well as lymphocytes, indicating that these receptors may serve an adhesive function in other tissue systems.
Other classes of adhesion receptors also contribute to lymphocyte-HEV interaction. The Integrin LFA-1 is important in the homotypic adhesion of activated lymphocytes and in the adhesion of T lymphocytes to endothelium (Haskard, D., et al., J. Immunol. 137:2901-2906, 1986). Antibodies to LFA-1 partially inhibit lymphocyte adhesion to peripheral lymph nodes in vitro and in vivo (Hamann, D. A., et al., J. Immunol. 140:693-699, 1988). However, in cell lines expressing high levels of the MEL-14 antigen, anti-LFA-1 treatment resulted in only minor reductions of HEV adhesion levels in contrast to treatment with MEL-14 antibody, which essentially eliminated adhesion to HEV. These results suggest that LFA-1 may play an accessory role in some lymphocyte-HEV interactions.
The study of HEV adhesion in the mouse has been facilitated by the exquisitely specific mab MEL-14. In the human and primate systems, the mab recognizing apparently related molecules often fail to block adhesion. The mechanism of organ specific HEV adhesion is unknown, as is the molecular basis for the functional diversity among these molecules.